Types of Water Catchments

Roof Rain Water Calculations

Catchment Sizes

Building Method

Case Study & Example

Further Reading/Videos


Updated March 7, 2011








Types of Water Catchments

Water catchment system have been around for several thousand years . It's nothing new and it comes in many forms, shapes and sizes. There are really only a few places to store the water once it is collected. It can be stored in a barrel of some sort, a lake, or in the soil itself.

If water is stored in a barrel, it can be used at any time. If the water is stored in the soil, some of it goes down to the water table and the rest is given up to plants, evaporation and evapotranspiration. If this system is used, it encourages plants to grow deeper roots. And the soil is a huge volume to store water. When the weather gets dry, water from deeper down in the soil moves to the surface. This is natures ways of regulating itself.

Below is a rain barrel water catchment. The container is opaque to prevent light from entering into the container. This prevents the water from going sour.

Here is a water catchment system on top of a chicken coop.

This is an interesting catchment system. It's normally used by artisans to make the landscape look really nice. Water collects in the dry bed and is absorbed into the soil.

Below is a picture of an Iron Age water catchment located in Israel. This is the entrance to a large hole/cave like room where the water is collected during the raining season and then is saved. There are many of these catchment systems throughout Jordon and Israel. Climbing down one can get scary.

Here is an example of a hidden catchment system. All of the rain is caught off the roof and is absorbed by the ground. The catchment system is located about 18 inches below the surface. The catchment holding area is made up of small 3/4 inch stones and has the volume of 27 cubic feet (or 1 cubic meter) The catchment system is located just to the right of the tree. There is irrigation for the grass, but it is not turned on during the summer. And the grass stays green like this. The tree takes advantage of the system as well.

Here is a simple and low cost way to add water catchments to your home. Replace the drain pipe from the gutters with a chain. The water flows down the chain into a box that is planted about 2 feet into the ground filled with 3/4 inch stones.

The bottom of the chain is attached to a box that is buried about 2 feet. Juncus is planted next to the catchment to prevent erosion.



Roof Rain Water Calculations

The calculation for rain harvesting is simple and straight forward. There are two places where rain water can be caught. Rain is caught off the roof or channeled into a holding area once it hits the ground.

The amount of water harvested off a roof is calculated by this formula.

Maximum water (gallons) = 7.48 gallons/ft3 * rainfall (in feet) * roof area (square feet)

In San Jose, CA we get about 12 inches of rain per year.

And my roof is about 20 feet X 100 feet. Plugging and chugging we get....

7.48 * 1 foot of rain * 20 feet X 100 feet = 14960 gallons of water per year for that roof. We normally don't get all that at one time. We get, maybe, 1/4 to 1 inch at a single rain event. If we would take the worst case and assume 1 inch of rain, we would get.

7.48 * 1/12 foot of rain * 20 feet X 100 feet = 1246 gallons in one storm for say a day (24 hours). In this case you would need a 1500 gallon barrel or container to hold the water or let the water be absorbed into the soil.

If you are going to absorb the water into to soil, you need to make sure the water absorption capacity is available. This is calculated by the following:

1246 gallons / 24 hours needs to be absorption rate. This means the soil has to absorb 52 gallon per hour. The catchment system needs to be designed to take 52 gallons in one hour. Therefore the surface area for the catchment system needs to be a minimum of 52 square feet. In the worst case, water needs to be absorbed by soil is 1 hour, then the catchment system needs to have 52 square feet of surface touching soil.

This number is not practical. Rain does not fall evenly through a rain storm. It comes in spirits. That means the catchment needs to handle gushes of water. A good rule of them is to design the system to twice the capacity. That means this rain water catchment system needs to be around 100 square feet.

This is all assuming you are catching 100% of the rain. And this is assuming your system does not have any leaks. The same calculations can be used for impervious surfaces like driveways and sidewalks.

Catchment Sizes and Shapes

It is not the size that matters but the shape. It is important to have the highest capacity for water to be absorbed by the soil to prevent flooding up the drain pipe or house.

In this case we bury a catchment system about 18 to 20 inches below the surface. It is important to maximize the contact surface area for water to penetrate into the soil as quickly as possible. This reduces the cost for the catchment system and saves in labor digging a deep hole.

Below is an example of 1 yard of 3/4 inch stone formed in a cube of 3 feet by 3 feet by 3 feet. This gives a volume of 27 cubic feet. The surface area is the outside face of the box. Surface contact is the place where the outer face of each side touches the soil. In this case there are 6 sides that touch the soil surface. The surface area is 54 square feet.

If we take the same volume of stones and change its shape, it is possible to double the surface area. In this case the volume is 36 cubic feet, but it has a surface area of 96 square feet. (And most of the contact area is on the bottom where gravity can help the soil absorb the water. This shape also helps in digging the hole. Instead of digging a hole 54 inches deep, you only have to go down 30 inches.


Building Method

Problem: Make a water catchment system harvesting all of the rain water off the roof. The water is absorbed into the ground. In this part of the country we get about 12 inches of rain per year. (That is not too much). The roof has about 2000 square feet surface area.

We start out by calculating the soil capacity to absorb water. The soil is mostly clay and compacted. A test was done to see how fast water is absorbed. A hole about 12 inches deep and 12 inches wide is dug. One gallon of water is placed in the hole. The water is observed. If the water disappears in an hour, that is the worst case cinereous. If it is absorbed in less than an hour, then you are in good standing.

The maximum time for water to be absorbed in the soil is 1 hour for 1 gallon. If it takes longer, then the size of the catchment is recommend to double the surface area. In this case the water disappears in 15 minutes.

In San Jose, CA we get about 12 inches of rain per year. But that is not all at one time. We get maximum about 1 inch per rain event. That means the system has to since about 1500 gallons of water over 24 hours. See the section on rain water calculation. This means the water catchment system needs to be around 100 square feet in surface area. Digging a hole about 6 feet by 6 feet will do the job.

In this building example we are digging a hole about 6 feet by 6 feet and 30 inches down. A 4 inch pipe is run from the down spout to the catchment. (That is viewed to the right). The slope of the pipe needs to be at least 1/4 inch drop per foot of pipe. That is a minimum of a 2% grade. The grade can be accomplished by using a leveling laser..


Place the HeNe laser at one end.

Place a stake at the other end. If the run length is 15 feet, that means the pipe has to drop about 4 inches at the other end. With a leveling laser, the far point can be positioned. Then switch to a single point laser to see the beam during the day.

The beam from the point laser is shined on the making stick.

Don't look into the laser at any time. It may not seem too bright, but it is what you don't see that destroys your eyes. Use a screen to view the laser beam. You can measure from the beam to the bottom of the trough to see if you have the right slope and you dug out enough.

Here is the finished catchment with the pipe. The white pipe is an irrigation system run.

When everything is dug out, you need to test the system to make sure the slope of the 4 inch pipe moves the water and the soil can absorb the water. That is done by taking a garden hose and propping it up on rain gutter and letting water run for a while. If all the water disappears in about 15 to 30 minutes, you are in really good shape. If not, you may need to double the size of your catchment. The best way to test this is to let it rain. See below.

Landscape fabric is installed on the outside surfaces of the catchment system to prevent mud from being created and filling in the spaces between the stones. The spaces between the stones is the volume where the water is stored while it is waiting to be absorbed into the soil.

Fill the volume with stones.

Cover the top of the catchment system and center the 4 inch pipe. This pipe has no holes in it.

Add the end cap. A cap is added to prevent borrowing animals from making trouble.

Insert it into the catchment.

Cover. Make sure to leave extra soil for compaction. Over time the area that was dug up will sink and settle.

This is a "before" picture of the catchment.

And this is the "after" picture. The underground square is just right of the tree.++-9-

Last, the drain pipe from the gutter is connected to the catchment system. A pool filter basket is used to collect large objects  and preventing plugging up the pipe. The pipe going to the left is a water drain from the aquaponics system to the left in the picture.

Here is a down view of the filter basket. You can see it collected some stuff off the roof.

That's it! This system has been in place for about 2 years. It has worked flawlessly and it has surprised me on how much water it stores.

Case Study

Here is an example of a neighbor who wants to install a hidden water catchment system in their front yard. The catchment system will be installed in the front yard just left of the front window. All of the roof rain water will be saved in the soil in the front yard. The front yard is going to be landscaped with all California native plants. The position of the water catchment is placed in accordance where native plants will need water to establish for the next two years.  After that, it's all bonus water.

In this case the owners removed two very tall palm trees. The arborist who did the work ground them up and placed them on the front yard. Palms are not the best things to use for soil fortification, but it is available and it is free. A series of Compost Tea applications will happen in the next few weeks to break down the material. With the combination of the Tea application and rain, the palm mulch will turn into something we can use quickly.

We are using what is on the land to fortify the soil and reuse what is considered waste. Since palm tree mulch has a lot of carbon and once the mulch is decomposed, the soil will be very good with fungi and protists.

We are preventing having to remove the top 12 or 18 inches of soil and replacing it with something that can grow things. We are rebuilding the soil on the site. It just takes time. Since the home owners are not in too much of a hurry, they are Ok with allowing nature reclaim their land. That trait in the neighbor is totally unheard of and is exceptional!


Here is the plan for the watering catchment system. This is a side view of the home modeled. You can see the placement of the system and how it moves water from the downspout to the catchment. Once I get good at using the software, I will complete the plans for the whole landscape.

All of the water from the front roof is drained through this down spout. The pool filter basket will be installed under this pipe.

And the 4 inch non-perforated corrugated pipe will extend to the left part of the uncovered soil. The plan is to water plants to the left and have water flow down towards the street.


In this case, the roof has an area of 20 feet by 80 feet. This will give use around 1000 gallons of water per event (24 hours). That means the soils needs to absorb all of the water within 24 hours. Therefore the catchment system needs to be around 5 feet by 6 feet and 1 foot deep.

To be continued....... (As of March 7, 2011)


Further Reading/Videos


Gardening Rhythms: Water Catchments Part 1

Gardening Rhythms: Water Catchments Part 2


Rainwater Harvesting for Dryland and Beyond, Brad Lancaster, Volumes 1 &2; Rainsource Press. Tucson, Arizona; Last published 2008


Water Catchement Systems